The selective oligomerization of isobutene is an important chemical reaction, particularly where the isobutene is contained in a refinery C4 hydrocarbon stream, such as Raffinate-1 and Raffinate-2.
For example, C4 linear olefins are an attractive feedstock for producing octenes with zeolite catalysts because, among other reasons, the resulting octenes have triple branching of less than about 5 wt %. However, when isobutene is present in significant quantities (>10 wt %) in the oligomerization feedstock, such as with Raffinate-1, the amount of triple-branched octenes increases to a level which is unacceptable for some end uses such as certain plasticizers. In the past, this problem was generally addressed by selectively reacting the isobutene with methanol to produce methyl t-butyl ether (MTBE). However, with the phase-out of MTBE because of environmental concerns, this reaction is no longer an attractive method of removing isobutene. As a result, interest has focused on selective dimerization of the isobutene to produce octenes useful as, for example, gasoline octane enhancers and as feedstocks for producing C9 aldehydes and/or alcohols.
Another use for the selective oligomerization of isobutene is in the purification of C4 olefin streams used in the alkylation of benzene to produce sec-butylbenzene, an important precursor in the production of phenol. Thus, even when present at only low levels (<5 wt %) in a C4 olefin stream, such as Raffinate-2, isobutene reacts with benzene to produce tert-butylbenzene. However, tert-butylbenzene is difficult to separate from sec-butylbenzene by distillation since the boiling points of the two butylbenzene isomers are very similar, 169° C. for tert-butylbenzene as compared with 173° C. for sec-butylbenzene. Moreover, tert-butylbenzene is known to be an inhibitor to the oxidation of sec-butylbenzene to the corresponding hydroperoxide, which is the first step in the conversion of sec-butylbenzene to phenol and methyl ethyl ketone.
One example of a process for the dimerization of isobutene is disclosed in U.S. Pat. No. 6,914,166, in which a C4 olefinic feedstock containing isobutene and n-butene(s) is contacted with dealuminated zeolite beta under conditions including a temperature below 50° C. effective to allow selective dimerization of isobutene to trimethylpentene(s).
U.S. Pat. No. 7,112,711 discloses a process for oligomerizing alkenes having from 3 to 6 carbon atoms, including isobutene, in the presence of a catalyst containing a zeolite of the MFS structure type. The process is carried out at a temperature comprised between 125 and 175° C. when the feedstock contains only alkenes with 3 carbon atoms and between 140 and 240° C. when the feedstock contains at least one alkene with 4 or more carbon atoms.
U.S. Patent Application Publication No. 2007/0213576, published Sep. 13, 2007, discloses a process for the dimerization of isobutene at a temperature in excess of 240° C. in the presence of a multi-dimensional molecular sieve catalyst containing at least one 10 or 12 ring channel, such as ZSM-57, ZSM-5, FAU, Beta, ZSM-12, mordenite, MCM-22 family zeolites, and mixtures thereof to produce a product low in triple-branched octenes.
According to the present invention, it has now been found that EMM-10, a novel molecular sieve of the MCM-22 family, is an effective catalyst for the low temperature selective oligomerization of isobutene in the presence of other C4 alkenes.